Calendering is an economic and highly efficient means to produce film and sheet from plastics. These films and sheets usually have a thickness ranging from about 1 mil (0.025 mm) to about 80 mils (2 mm). They are readily thermoformed into various shapes that are used for a variety of packaging applications. For example, calendered poly(vinyl chloride) film or sheet (“PVC”) can be used in a wide range of applications including pool liners, graphic arts, transaction cards, security cards, veneers, wall coverings, greenhouse glazing, book bindings, folders, floor tiles, and products that are printed or decorated or laminated in a secondary operation.
Although PVC has been used for over sixty years, it presents disposal and environmental problems that have created a need for alternative, environmentally-friendly materials. For example, PVC emits toxic byproducts when incinerated and is persistent in the environment. Although PVC compositions are by far the largest segment of the calendered film and sheet business, small amounts of other thermoplastic polymers such as thermoplastic rubbers, certain polyurethanes, talc-filled polypropylene, acrylonitrile/-butadiene/styrene terpolymers (abbreviated herein as “ABS”), and chlorinated poly-ethylene are sometimes processed by calendering methods. Attempts to calender polyester polymers such as poly(ethylene terephthalate) (abbreviated herein as “PET”) or poly(1,4-butylene terephthalate) (abbreviated herein as “PBT”) have not been successful. For example, PET polymers with inherent viscosity values of about 0.6 dL/g have insufficient melt strength to perform properly on the calendering rolls. PET also crystallizes rapidly and uncontrollably when fed to calendering rolls at typical processing temperatures and forms a non-homogeneous mass which is unsuitable for further processing. This non-homogeneous mass causes undesirable, high forces on the calender bearings. During processing, the tendency of polyester polymers to hydrolyze on rolls open to ambient conditions and at the high temperatures required to produce polymer melts also can be a concern. Typical PET polymers without the inclusion of process lubricants or internal release additives also have a tendency to stick to the calendering rolls at typical processing temperatures. The calendering of various copolyester compositions and several approaches to these problems has been described, for example, in U.S. Pat. Nos. 5,998,005; 6,068,910; 6,551,688; U.S. patent application Ser. No. 10/086,905; Japan Patent Application No.'s 8-283547; 7-278418; 9-217014; 2002-53740; 10-363-908; 2002-121362; 2003-128894; 11-158358; European Patent Application No. 1 375 556 A2; and World Patent Application No. 02/28967. Although some of these difficulties can be avoided by the careful selection of polymer properties, additives, and processing conditions, calendering of polyesters can be troublesome or heretofore impossible or impractical.
Conventional processing of polyesters into film or sheet involves extruding a polyester melt through a manifold of a flat die. Manual or automatic die lip adjustment is used to control thickness across a web of material. Water-cooled chill rolls are used to quench the molten web and impart a smooth surface finish. Extrusion processes, while producing film and sheet of excellent quality, do not have the throughput and economic advantages that are provided by calendering processes. Also, the gauge tolerance in a calender process is better than in extrusion. Moreover, extruded films produced from aliphatic-aromatic polyesters such as, for example, ECOFLEX® Copolyester (available from BASF Corporation), other similar biodegradable resins, and blends of these resins typically have poor optical properties, i.e., generally not clear, but may exhibit improved clarity on contact with surfaces. The poor clarity, in part, is the result of anti-block additives that are required to successfully process these resins using the conventional processing technologies such as melt-casting and melt-blowing. Such poor clarity makes these films unacceptable for many commercial applications. Films prepared by conventional methods from biodegradable polymers also tend to have poor strength and tear easily. These films, therefore, lack the toughness necessary for many applications such as, for example, trash bags.
Polymers experience a thermal transition known as the glass transition temperature or Tg. Articles made from polymers that exhibit a Tg at or below room temperature, typically, are considered flexible. In general, the further the Tg is below room temperature, the more flexible the polymer will be. For commercial applications requiring polymers of higher flexibility and increased soft feel, a lower Tg is usually obtained by utilizing or designing polymers with an inherently lower Tg such as, for example, polyethylene, or using an additive such as, for example, a plasticizer, that can reduce the Tg to the desired temperature. For example, polyesters typically require the presence of a plasticizer to attain the flexibility needed for many film applications.
The preparation of polymers with an inherently lower Tg can be accomplished with the proper selection of monomers. In some cases, however, the resulting polymer will lose important characteristics. One of these relates to the surface character of the film or article. Typically, as Tg is lowered, there is an increase of surface tackiness that results in an increase in adhesion to surfaces. Consequently, articles and films made with low Tg materials will stick to themselves, even to the point of coalescing or fusing the articles or films into one mass. One way to overcome this problem is to incorporate an “anti-blocking” additive such as, for example, a mineral or higher Tg polymer, that presents itself at the polymer surface and provides a new surface on the film or article with the adhesion characteristics of the additive. Antiblocking additives, however, sometimes reduce the clarity of the calendered film and are often undesirable for many packaging applications.
The presence of a plasticizer in calendered films also is frequently undesirable in some applications. For example, the concentration of plasticizer in polyester films may gradually decrease, either through volatile or extractive loss, which may cause an unacceptable, gradual change in the physical properties of the films. The plasticizer also may cause contamination of the vapor or liquid in contact with the film. Many plasticizers degrade slowly in the environment and, thus, present environmental concerns in addition to the litter problem discussed above. Hence, producing a flexible film without plasticizer is desirable and, eventually, may be required in some film applications.
The shortcomings discussed above, therefore, have created a need for flexible, calendered films and sheets that exhibit high clarity, high thermal resistance, and superior toughness that do not require added plasticizers or anti-blocking agents. Further, there is a need for high clarity, thermally resistant, flexible films that are biodegradable in the environment. Such films have applications as packaging materials, waste and trash bags, and agricultural films as well as many other applications which require the advantages noted above.